Purpose:
Meet our preseason drive base Slippery Biscuit. In the past we have designed a rectangular chassis and it has worked well for us. Because of the nature of last year's open field, we ran into major issues with friction pinning. These issues became more prominent when we competed against robots with 3 CIM transmissions versus our 2 CIM transmissions. So, for this year's preseason project, we choose to design and build a drive base that contained some good solutions to friction pinning to determine how well they work and how to build them during build season. This design has redundant friction pin solutions. The link the the CAD model is
https://drive.google.com/open?id=0By...VkE&authuser=0
Features:
The drive base features a sheet metal, 6 wheel tank drive with a dropped center wheel for improved maneuverability. It has an octagon shape with an outward slope of 25 degrees yielding a larger platform and a geometry that allows steering out of a friction pin situation. This is inspired by FRC Team 971, Spartan Robotics. All wheels are VEXPro 4 inch traction wheels using Blue Nitrile Rough Top tread. The front and back wheels are 1 inch wide riding on dead axles while the center wheels are 2 inches wide on a live axle. The center wheels are driven by a chain off the transmissions. The chain is tensioned using a cam that positions transmission. The outer wheels are driven off the center shaft using 9mm wide, 5mm pitch HTD belts on 42 tooth VersaPulleys. The belts are tensioned by adjusting the position of the dead axles.
The transmissions have custom water jetted and milled aluminum bearing plates and standard Andymark Supershifter gears/shafts with 3 in-line CIMs.$@# A$@#dog-shifter enables a high gear ratio of 4.82:1 (16 fps) and a low gear ratio of 12.35:1 (6 fps).$@# Speeds are calculated at 81% of CIM free speed. High speed reduction is 12:40, 28:35, and 19:22. Low speed reduction is 12:40, 15:48, and 19:22. The only custom gear in the transmission is 3-D printed idler gear that drives a Grayhill encoder.
In between the outer and center wheels are drop omni-wheels that are pneumatically deployed. This enables the robot to pivot out of friction pinning situations. This feature is inspired by FRC Team 118, Robonauts. This design features a departure from our traditional two-color Velcro bumper system. Here we are studying a single piece bumper system that is easily removed, yet rugged under heavy defense. Over-centering toggle clamps retain the bumper by pinching a flange attached to the bumper against the top of the chassis.
Things we would like to investigate further:
Instead of using a chain to drive our center shaft, we would like to drive it using gears. We will not place the air tanks in the frame. We thought it would be a good use of space since we wanted a box frame in the front and back, but the tanks are difficult to service if a leak occurs. As for belts, they are nice and light but they are difficult to remove/replace.